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NAME

       ioctl_userfaultfd - create a file descriptor for handling page faults in user space

LIBRARY

       Standard C library (libc, -lc)

SYNOPSIS

       #include <linux/userfaultfd.h>  /* Definition of UFFD* constants */
       #include <sys/ioctl.h>

       int ioctl(int fd, int cmd, ...);

DESCRIPTION

       Various ioctl(2) operations can be performed on a userfaultfd object (created by a call to
       userfaultfd(2)) using calls of the form:

           ioctl(fd, cmd, argp);
       In the above, fd is a file descriptor referring to a userfaultfd object, cmd is one of the
       commands listed below, and argp is a pointer to a data structure that is specific to cmd.

       The various ioctl(2) operations are described below.  The UFFDIO_API, UFFDIO_REGISTER, and
       UFFDIO_UNREGISTER operations are used to configure userfaultfd behavior.  These operations
       allow  the caller to choose what features will be enabled and what kinds of events will be
       delivered to the application.  The  remaining  operations  are  range  operations.   These
       operations enable the calling application to resolve page-fault events.

   UFFDIO_API
       (Since Linux 4.3.)  Enable operation of the userfaultfd and perform API handshake.

       The argp argument is a pointer to a uffdio_api structure, defined as:

           struct uffdio_api {
               __u64 api;        /* Requested API version (input) */
               __u64 features;   /* Requested features (input/output) */
               __u64 ioctls;     /* Available ioctl() operations (output) */
           };

       The api field denotes the API version requested by the application.

       The  kernel  verifies that it can support the requested API version, and sets the features
       and ioctls fields to bit masks representing all the available  features  and  the  generic
       ioctl(2) operations available.

       Before  Linux  4.11,  the  features  field  must be initialized to zero before the call to
       UFFDIO_API, and zero (i.e., no feature bits) is placed in the features field by the kernel
       upon return from ioctl(2).

       Starting  from  Linux  4.11,  the  features  field  can  be used to ask whether particular
       features are supported and explicitly enable userfaultfd features  that  are  disabled  by
       default.  The kernel always reports all the available features in the features field.

       To  enable  userfaultfd  features  the  application should set a bit corresponding to each
       feature it wants to enable in  the  features  field.   If  the  kernel  supports  all  the
       requested  features  it  will  enable  them.   Otherwise  it  will  zero  out the returned
       uffdio_api structure and return EINVAL.

       The following feature bits may be set:

       UFFD_FEATURE_EVENT_FORK (since Linux 4.11)
              When this feature is enabled, the userfaultfd  objects  associated  with  a  parent
              process  are duplicated into the child process during fork(2) and a UFFD_EVENT_FORK
              event is delivered to the userfaultfd monitor

       UFFD_FEATURE_EVENT_REMAP (since Linux 4.11)
              If this feature is enabled,  when  the  faulting  process  invokes  mremap(2),  the
              userfaultfd monitor will receive an event of type UFFD_EVENT_REMAP.

       UFFD_FEATURE_EVENT_REMOVE (since Linux 4.11)
              If  this  feature  is  enabled, when the faulting process calls madvise(2) with the
              MADV_DONTNEED or MADV_REMOVE advice  value  to  free  a  virtual  memory  area  the
              userfaultfd monitor will receive an event of type UFFD_EVENT_REMOVE.

       UFFD_FEATURE_EVENT_UNMAP (since Linux 4.11)
              If  this feature is enabled, when the faulting process unmaps virtual memory either
              explicitly with munmap(2), or implicitly during either mmap(2)  or  mremap(2),  the
              userfaultfd monitor will receive an event of type UFFD_EVENT_UNMAP.

       UFFD_FEATURE_MISSING_HUGETLBFS (since Linux 4.11)
              If  this  feature bit is set, the kernel supports registering userfaultfd ranges on
              hugetlbfs virtual memory areas

       UFFD_FEATURE_MISSING_SHMEM (since Linux 4.11)
              If this feature bit is set, the kernel supports registering userfaultfd  ranges  on
              shared  memory areas.  This includes all kernel shared memory APIs: System V shared
              memory, tmpfs(5), shared mappings of /dev/zero, mmap(2) with  the  MAP_SHARED  flag
              set, memfd_create(2), and so on.

       UFFD_FEATURE_SIGBUS (since Linux 4.14)
              If  this  feature  bit  is set, no page-fault events (UFFD_EVENT_PAGEFAULT) will be
              delivered.  Instead, a  SIGBUS  signal  will  be  sent  to  the  faulting  process.
              Applications  using  this feature will not require the use of a userfaultfd monitor
              for processing memory accesses to the regions registered with userfaultfd.

       UFFD_FEATURE_THREAD_ID (since Linux 4.14)
              If this feature bit is set, uffd_msg.pagefault.feat.ptid will be set to the faulted
              thread ID for each page-fault message.

       UFFD_FEATURE_MINOR_HUGETLBFS (since Linux 5.13)
              If  this  feature bit is set, the kernel supports registering userfaultfd ranges in
              minor mode on hugetlbfs-backed memory areas.

       UFFD_FEATURE_MINOR_SHMEM (since Linux 5.14)
              If this feature bit is set, the kernel supports registering userfaultfd  ranges  in
              minor mode on shmem-backed memory areas.

       The returned ioctls field can contain the following bits:

       1 << _UFFDIO_API
              The UFFDIO_API operation is supported.

       1 << _UFFDIO_REGISTER
              The UFFDIO_REGISTER operation is supported.

       1 << _UFFDIO_UNREGISTER
              The UFFDIO_UNREGISTER operation is supported.

       This  ioctl(2)  operation returns 0 on success.  On error, -1 is returned and errno is set
       to indicate the error.  Possible errors include:

       EFAULT argp refers to an address that is outside the calling process's accessible  address
              space.

       EINVAL The userfaultfd has already been enabled by a previous UFFDIO_API operation.

       EINVAL The  API version requested in the api field is not supported by this kernel, or the
              features field passed to the kernel includes feature bits that are not supported by
              the current kernel version.

   UFFDIO_REGISTER
       (Since  Linux  4.3.)   Register  a  memory address range with the userfaultfd object.  The
       pages in the range must be "compatible".  Please refer to the list of register modes below
       for the compatible memory backends for each mode.

       The argp argument is a pointer to a uffdio_register structure, defined as:

           struct uffdio_range {
               __u64 start;    /* Start of range */
               __u64 len;      /* Length of range (bytes) */
           };

           struct uffdio_register {
               struct uffdio_range range;
               __u64 mode;     /* Desired mode of operation (input) */
               __u64 ioctls;   /* Available ioctl() operations (output) */
           };

       The range field defines a memory range starting at start and continuing for len bytes that
       should be handled by the userfaultfd.

       The mode field defines the  mode  of  operation  desired  for  this  memory  region.   The
       following values may be bitwise ORed to set the userfaultfd mode for the specified range:

       UFFDIO_REGISTER_MODE_MISSING
              Track page faults on missing pages.  Since Linux 4.3, only private anonymous ranges
              are compatible.  Since Linux 4.11, hugetlbfs and  shared  memory  ranges  are  also
              compatible.

       UFFDIO_REGISTER_MODE_WP
              Track  page  faults  on  write-protected  pages.   Since  Linux  5.7,  only private
              anonymous ranges are compatible.

       UFFDIO_REGISTER_MODE_MINOR
              Track minor page faults.  Since Linux 5.13, only hugetlbfs ranges  are  compatible.
              Since Linux 5.14, compatibility with shmem ranges was added.

       If  the operation is successful, the kernel modifies the ioctls bit-mask field to indicate
       which ioctl(2) operations are available for the specified range.  This returned  bit  mask
       can contain the following bits:

       1 << _UFFDIO_COPY
              The UFFDIO_COPY operation is supported.

       1 << _UFFDIO_WAKE
              The UFFDIO_WAKE operation is supported.

       1 << _UFFDIO_WRITEPROTECT
              The UFFDIO_WRITEPROTECT

       1 << _UFFDIO_ZEROPAGE
              The UFFDIO_ZEROPAGE operation is supported.

       1 << _UFFDIO_CONTINUE
              The UFFDIO_CONTINUE operation is supported.

       This  ioctl(2)  operation returns 0 on success.  On error, -1 is returned and errno is set
       to indicate the error.  Possible errors include:

       EBUSY  A mapping in the specified range is registered with another userfaultfd object.

       EFAULT argp refers to an address that is outside the calling process's accessible  address
              space.

       EINVAL An  invalid  or  unsupported bit was specified in the mode field; or the mode field
              was zero.

       EINVAL There is no mapping in the specified address range.

       EINVAL range.start or range.len is not a multiple of the system page size;  or,  range.len
              is zero; or these fields are otherwise invalid.

       EINVAL There as an incompatible mapping in the specified address range.

   UFFDIO_UNREGISTER
       (Since  Linux 4.3.)  Unregister a memory address range from userfaultfd.  The pages in the
       range must be "compatible" (see the description of UFFDIO_REGISTER.)

       The address range to unregister is specified in the uffdio_range structure pointed  to  by
       argp.

       This  ioctl(2)  operation returns 0 on success.  On error, -1 is returned and errno is set
       to indicate the error.  Possible errors include:

       EINVAL Either the start or the len field of the ufdio_range structure was not  a  multiple
              of  the system page size; or the len field was zero; or these fields were otherwise
              invalid.

       EINVAL There as an incompatible mapping in the specified address range.

       EINVAL There was no mapping in the specified address range.

   UFFDIO_COPY
       (Since Linux  4.3.)   Atomically  copy  a  continuous  memory  chunk  into  the  userfault
       registered  range  and  optionally wake up the blocked thread.  The source and destination
       addresses and the number of bytes to copy are specified by the src, dst, and len fields of
       the uffdio_copy structure pointed to by argp:

           struct uffdio_copy {
               __u64 dst;    /* Destination of copy */
               __u64 src;    /* Source of copy */
               __u64 len;    /* Number of bytes to copy */
               __u64 mode;   /* Flags controlling behavior of copy */
               __s64 copy;   /* Number of bytes copied, or negated error */
           };

       The  following value may be bitwise ORed in mode to change the behavior of the UFFDIO_COPY
       operation:

       UFFDIO_COPY_MODE_DONTWAKE
              Do not wake up the thread that waits for page-fault resolution

       UFFDIO_COPY_MODE_WP
              Copy the page with read-only permission.  This allows the user  to  trap  the  next
              write  to  the  page, which will block and generate another write-protect userfault
              message.   This  is  used   only   when   both   UFFDIO_REGISTER_MODE_MISSING   and
              UFFDIO_REGISTER_MODE_WP modes are enabled for the registered range.

       The  copy  field  is  used  by  the kernel to return the number of bytes that was actually
       copied, or an error (a negated errno-style value).  If the value returned in copy  doesn't
       match the value that was specified in len, the operation fails with the error EAGAIN.  The
       copy field is output-only; it is not read by the UFFDIO_COPY operation.

       This ioctl(2) operation returns 0 on success.  In this case, the entire area  was  copied.
       On error, -1 is returned and errno is set to indicate the error.  Possible errors include:

       EAGAIN The  number  of  bytes copied (i.e., the value returned in the copy field) does not
              equal the value that was specified in the len field.

       EINVAL Either dst or len was not a  multiple  of  the  system  page  size,  or  the  range
              specified by src and len or dst and len was invalid.

       EINVAL An invalid bit was specified in the mode field.

       ENOENT (since Linux 4.11)
              The  faulting  process has changed its virtual memory layout simultaneously with an
              outstanding UFFDIO_COPY operation.

       ENOSPC (from Linux 4.11 until Linux 4.13)
              The faulting process has exited at the time of a UFFDIO_COPY operation.

       ESRCH (since Linux 4.13)
              The faulting process has exited at the time of a UFFDIO_COPY operation.

   UFFDIO_ZEROPAGE
       (Since Linux 4.3.)  Zero out a memory range registered with userfaultfd.

       The requested range is specified by the  range  field  of  the  uffdio_zeropage  structure
       pointed to by argp:

           struct uffdio_zeropage {
               struct uffdio_range range;
               __u64 mode;     /* Flags controlling behavior of copy */
               __s64 zeropage; /* Number of bytes zeroed, or negated error */
           };

       The  following  value  may  be  bitwise  ORed  in  mode  to  change  the  behavior  of the
       UFFDIO_ZEROPAGE operation:

       UFFDIO_ZEROPAGE_MODE_DONTWAKE
              Do not wake up the thread that waits for page-fault resolution.

       The zeropage field is used by the kernel to return the number of bytes that  was  actually
       zeroed,  or  an  error  in  the  same manner as UFFDIO_COPY.  If the value returned in the
       zeropage field doesn't match the value that was  specified  in  range.len,  the  operation
       fails  with  the  error  EAGAIN.  The zeropage field is output-only; it is not read by the
       UFFDIO_ZEROPAGE operation.

       This ioctl(2) operation returns 0 on success.  In this case, the entire area  was  zeroed.
       On error, -1 is returned and errno is set to indicate the error.  Possible errors include:

       EAGAIN The  number  of  bytes zeroed (i.e., the value returned in the zeropage field) does
              not equal the value that was specified in the range.len field.

       EINVAL Either range.start or range.len was not a multiple of  the  system  page  size;  or
              range.len was zero; or the range specified was invalid.

       EINVAL An invalid bit was specified in the mode field.

       ESRCH (since Linux 4.13)
              The faulting process has exited at the time of a UFFDIO_ZEROPAGE operation.

   UFFDIO_WAKE
       (Since  Linux  4.3.)   Wake up the thread waiting for page-fault resolution on a specified
       memory address range.

       The UFFDIO_WAKE operation is used in  conjunction  with  UFFDIO_COPY  and  UFFDIO_ZEROPAGE
       operations  that  have  the UFFDIO_COPY_MODE_DONTWAKE or UFFDIO_ZEROPAGE_MODE_DONTWAKE bit
       set in the mode  field.   The  userfault  monitor  can  perform  several  UFFDIO_COPY  and
       UFFDIO_ZEROPAGE  operations  in  a  batch  and then explicitly wake up the faulting thread
       using UFFDIO_WAKE.

       The argp argument is a pointer to a uffdio_range structure (shown  above)  that  specifies
       the address range.

       This  ioctl(2)  operation returns 0 on success.  On error, -1 is returned and errno is set
       to indicate the error.  Possible errors include:

       EINVAL The start or the len field of the ufdio_range structure was not a multiple  of  the
              system page size; or len was zero; or the specified range was otherwise invalid.

   UFFDIO_WRITEPROTECT (Since Linux 5.7)
       Write-protect  or  write-unprotect  a  userfaultfd-registered memory range registered with
       mode UFFDIO_REGISTER_MODE_WP.

       The argp argument is a pointer to a uffdio_range structure as shown below:

           struct uffdio_writeprotect {
               struct uffdio_range range; /* Range to change write permission*/
               __u64 mode;                /* Mode to change write permission */
           };

       There are two mode bits that are supported in this structure:

       UFFDIO_WRITEPROTECT_MODE_WP
              When this mode bit is set, the ioctl will be a  write-protect  operation  upon  the
              memory  range specified by range.  Otherwise it will be a write-unprotect operation
              upon the specified range, which can be used to resolve a userfaultfd  write-protect
              page fault.

       UFFDIO_WRITEPROTECT_MODE_DONTWAKE
              When  this  mode  bit  is  set, do not wake up any thread that waits for page-fault
              resolution   after   the   operation.    This   can   be    specified    only    if
              UFFDIO_WRITEPROTECT_MODE_WP is not specified.

       This  ioctl(2)  operation returns 0 on success.  On error, -1 is returned and errno is set
       to indicate the error.  Possible errors include:

       EINVAL The start or the len field of the ufdio_range structure was not a multiple  of  the
              system page size; or len was zero; or the specified range was otherwise invalid.

       EAGAIN The process was interrupted; retry this call.

       ENOENT The  range  specified in range is not valid.  For example, the virtual address does
              not exist, or not registered with userfaultfd write-protect mode.

       EFAULT Encountered a generic fault during processing.

   UFFDIO_CONTINUE
       (Since Linux 5.13.)  Resolve a minor page fault  by  installing  page  table  entries  for
       existing pages in the page cache.

       The argp argument is a pointer to a uffdio_continue structure as shown below:

           struct uffdio_continue {
               struct uffdio_range range;
                              /* Range to install PTEs for and continue */
               __u64 mode;    /* Flags controlling the behavior of continue */
               __s64 mapped;  /* Number of bytes mapped, or negated error */
           };

       The  following  value  may  be  bitwise  ORed  in  mode  to  change  the  behavior  of the
       UFFDIO_CONTINUE operation:

       UFFDIO_CONTINUE_MODE_DONTWAKE
              Do not wake up the thread that waits for page-fault resolution.

       The mapped field is used by the kernel to return the number of bytes  that  were  actually
       mapped,  or  an  error  in  the  same manner as UFFDIO_COPY.  If the value returned in the
       mapped field doesn't match the value that was specified in range.len, the operation  fails
       with  the  error  EAGAIN.   The  mapped  field  is  output-only;  it  is  not  read by the
       UFFDIO_CONTINUE operation.

       This ioctl(2) operation returns 0 on success.  In this case, the entire area  was  mapped.
       On error, -1 is returned and errno is set to indicate the error.  Possible errors include:

       EAGAIN The  number of bytes mapped (i.e., the value returned in the mapped field) does not
              equal the value that was specified in the range.len field.

       EINVAL Either range.start or range.len was not a multiple of  the  system  page  size;  or
              range.len was zero; or the range specified was invalid.

       EINVAL An invalid bit was specified in the mode field.

       EEXIST One or more pages were already mapped in the given range.

       ENOENT The  faulting  process has changed its virtual memory layout simultaneously with an
              outstanding UFFDIO_CONTINUE operation.

       ENOMEM Allocating memory needed to setup the page table mappings failed.

       EFAULT No existing page could be found in the page cache for the given range.

       ESRCH  The faulting process has exited at the time of a UFFDIO_CONTINUE operation.

RETURN VALUE

       See descriptions of the individual operations, above.

ERRORS

       See descriptions of the individual operations, above.  In addition, the following  general
       errors can occur for all of the operations described above:

       EFAULT argp does not point to a valid memory address.

       EINVAL (For  all  operations  except UFFDIO_API.)  The userfaultfd object has not yet been
              enabled (via the UFFDIO_API operation).

STANDARDS

       These ioctl(2) operations are Linux-specific.

BUGS

       In order to detect available userfault features and enable some subset of  those  features
       the  userfaultfd  file descriptor must be closed after the first UFFDIO_API operation that
       queries features availability and reopened before the  second  UFFDIO_API  operation  that
       actually enables the desired features.

EXAMPLES

       See userfaultfd(2).

SEE ALSO

       ioctl(2), mmap(2), userfaultfd(2)

       Documentation/admin-guide/mm/userfaultfd.rst in the Linux kernel source tree